1. Field of the Invention
This invention relates generally to plasma arc torches, and more particularly to the design and construction of an improved collimator component of such plasma arc torches.
2. Discussion of the Prior Art
Plasma arc torches are known in the prior art and comprise a device which can efficiently convert electrical energy into heat energy. Plasma arc torches, as the name implies, generate a plasma plume exhibiting a high specific enthalpy coupled with low gas requirements. As is set out in the Camacho et al. U.S. Pat. No. 4,559,439, there are basically two types of plasma arc torches. The first type is referred to as a non-transferred arc torch and the second is referred to as transferred arc torches. In the non-transferred arc type, there is a rear electrode, a front electrode-and a gas vortex generator that is coaxially placed between the front and rear electrodes. This assembly is contained within a water-cooled housing along with other components necessary for generating an electrical arc. The arc extends from the rear electrode past the gas vortex generator location and to an attachment point on the front electrode.
In the transferred-arc type of plasma generator, a collimating nozzle is mounted in coaxial alignment with the rear electrode and vortex generator. In this type of operation, the electrical arc attaches between the rear electrode and an external work piece that is being worked upon after passing through the collimating nozzle.
Transferred arc generators are described in U.S. Pat. No. 3,194,941 to Baird, and in U.S. Pat. No. 3,818,174 to Camacho. The present invention is directed to an improved collimator for a transferred arc-type plasma arc torch and is deemed to be an improvement over the prior art, such as the collimator shown in FIGS. 1-3 of the accompanying drawings.
Referring to FIG. 1, there is shown a conventional, prior art plasma torch of the transferred arc-type. It is indicated generally by numeral 10 and includes an outer steel shroud 12 having a proximal end 14 and a distal end 16. The shroud surrounds various internal components of the torch, including a rear electrode 18, a gas vortex generator 20 and other tubular structures that create a cooling water passage leading to a collimator member 22 that is threadedly attached into the distal end 16 of the shroud 12 and a passage for returning the heated cooling water to an outlet port. Tubing (not shown) connects to a water inlet stub 24 and after traversing the water passages in the torch body and the collimator, the heated water exits the torch at a port 26. Details of the water circulation path for a plasma arc torch are more clearly set out and explained in the Hanus et al. U.S. Pat. No. 5,362,939 and, hence, need not be repeated here. The gas for the plasma arc torch is applied under pressure to an inlet port 28 and it passes through annular channel isolated from the incoming and outgoing water channels, ultimately reaching the gas vortex generator 20. A high positive voltage is also applied to the water inlet stub 24 and the negative terminal of the power supply connects to the work piece 30.
The gas injected into port 28 becomes ionized and is rendered plasma by the arc 32 and is injected onto the work piece 30. The collimator 22 includes a longitudinal bore having a frusto-conical taper 34 and serves to concentrate the plasma into a beam, focusing intense heat that speeds up melting of and chemical reaction to the work piece in a furnace in which the plasma torch is installed.
Keeping in mind that the exposed toroidal face 36 of the collimator 22 is exposed to corrosive chemicals given off from the melting/gasification of the work material 30 as well as to secondary arcs, especially in the tapered zone 34 of the collimator, it is imperative that the collimator not be allowed to deteriorate to the point where cooling water can escape the normal channels provided in the torch and flow out onto the work piece that may typically be at a temperature of 2000° F. or more. Resulting superheated steam could create an explosive force within the confines of the plasma arc heated furnace. To avoid such an event, it becomes necessary to shut down the process and replace the collimator at relatively frequent intervals.
Referring to FIG. 2, there is shown a perspective view from the side of the prior art collimator 22 of FIG. 1. It is seen to comprise a holder member 38 having a generally cylindrical outer wall that is machined along a top edge portion with a flat surface, as at 40, forming a hexagonal pattern that allows the holder member to be grasped by jaws of a wrench and screwed into the threaded distal end of the torch shroud 12. The threads on the holder member are identified by numeral 42 in FIG. 2. The holder member 38 is preferably machined out from a generally cylindrical copper alloy billet, the particular copper alloy being a good electrical and thermal conductor.
Located directly below the threaded zone 42 on the holder member is a plurality of bores, as at 44, the bores being regularly spaced circumferentially about the periphery of the holder member. An integrally formed annular collar 46 is provided at the proximal end of the collimator.
FIG. 3 is a longitudinal, cross-sectional view taken through the center of the prior art collimator assembly. Here it can be seen that the holder member 38 has a central longitudinal bore 48 and a counterbore 50 that is formed inwardly from a face surface 52 of the holder member. Further, it can be seen that the radial bores 44 are in fluid communication with the central bore 48.
The prior art collimator 22 further includes a tubular insert 54 machined from a copper alloy billet. It has a central lumen 56 and an outer wall 58 whose diameter is dimensioned to fit within the central bore 48 of the holder member with a predetermined clearance space between the wall defining the central bore of the holder member and the outer diameter of the tubular insert. The insert is also formed with a circular plate-like flange 60 at its distal end that surrounds the lumen 56. Further, the cross-sectional view of FIG. 3 shows that the lumen 56 has a frusto-conical tapered portion 62 leading to a face surface 64 of the flange 60.
In the prior art collimator assembly shown in FIG. 3, with the tubular insert 54 disposed within the bore 48 of the holder member and with the flange 60 inserted into the counterbore 50, the joint between the periphery of the flange 60 and the wall of the counterbore 50 is suitably welded, preferably e-beam welded. Likewise, the joint between the collar 46 of the holder member and a portion of the exterior wall of the tubular insert are designed to fit together with a close tolerance and this joint is also e-beam welded.
As is explained in the Hanus, et al. '939 patent, supra, cooling water is made to flow through a first annular passageway in the torch housing, through the radial bores 44 of the collimator and through the clearance space between the bore 48 and the outer tubular wall 58 of the insert 54 and from there through radial bores 66 and out through an annular port to another passageway contained within the housing 12 and leading to the water outlet port 26 seen in FIG. 1.
The weld made at the joint between the counterbore 50 and the periphery of the flange 60 have proven to be problematic. Extensive corrosive action from the furnace gases corrodes the material on either side of this weld ring and of the e-beam weld itself The life of the collimator is thereby limited by either the integrity and precision of the e-beam weld itself or by the loss of material due to corrosion. This corrosive loss of material may be a result of both galvanic and non-galvanic corrosion. The galvanic corrosion, of course, is due to the presence of dissimilar materials in contact within an electrically conductive medium, such as the gas given off by the reaction of the arc flame with the work piece. The non-galvanic, standard corrosion is due to chemical reaction between the corrosive gases given off by vaporization of the work piece within the plasma arc heated furnace.
As is apparent from FIG. 3, failure of the e-beam weld, whether due to formation of a poor weld or because of corrosion, can lead to significant leakage of cooling water through the failed joint. To avoid this potentially harmful condition, the collimator component of the plasma arc torch must be replaced at frequent intervals before significant corrosion can occur, forcing a shut-down of the reactor furnace and attendant loss of production.
A need, therefore, exists for a collimator design having an increased working life and safety improvements over the prior art. The present invention satisfies this need.